Solid-state lasers operating in the 1.4 μm to 1.8 μm eye-safe wavelength range, which falls into the infrared atmospheric transmission window, have recently been the subject of intense research and development efforts. Applications of such lasers include range finding, illumination for long range target identification using gated imaging, eye-safe 3-D imaging LADAR (laser detection and ranging), remote sensing, etc.
In recent years, the main options considered for lasers in the eye-safe wavelength range were directed at down shifting a shorter wavelength radiation using a nonlinear conversion process such as an optical parametric oscillator (OPO) or Raman conversion. The OPO and Raman conversion methods suffer from a number of drawbacks including optical system complexity and degraded reliability due to, for example, potential damage to a nonlinear crystal used for the OPO or beam quality degradation in the case of Raman conversion. Furthermore, both methods generally require a high peak power (i.e., short pulse) pump laser for efficient radiation conversion. Consequently, both methods do not lend themselves well to quasi-continuous wave (CW) or long pulse applications. Another alternative method for obtaining laser radiation in the eye-safe wavelength range is direct emission from semiconductor lasers, the main drawback of which is a poor beam quality. Flashlamp pumped Erbium (Er),Ytterbium (Yb)-doped glass lasers are frequently used for producing eye-safe laser radiation, but the efficiency of such lasers is normally low (especially in a Q-switched regime due to cross-relaxation mechanisms, which limit stored energies).
It is generally accepted, that an efficient source based on a resonantly pumped in-band solid state laser is a preferred technology path. Resonant pumping of rare-earth-doped solid state lasers using appropriate pump lasers known in the art as linear down-conversion is a viable way of obtaining high-efficiency, multiple-wavelength, high average power sources. Spectral diversity in laser materials may be greatly extended through laser pumping that “shifts” the laser energy downward. Resonant pumping leads to high conversion efficiencies by permitting access to energy states difficult or impossible to pump with incoherent sources. Therefore, neither long lifetimes nor broadband absorption are required. Additionally, radiative upper and lower laser levels result in very low heat loading thereby allowing high average power operation.
The Erbium 4I13/2-4I15/2 laser transition has been a popular choice in numerous different hosts for generating laser radiation in the eye-safe wavelength range. In this transition the lower laser level is in the same energy manifold as the ground level, which leads to a three-level laser configuration at room temperature with high thresholds and low efficiency operation. An Erbium ion in a variety of hosts has also lased on the 4S3/2-4I15/2 laser transition around 1.73 μm, which behaves more nearly as a four-level transition that can be operated at room temperature. Resonant pumping of the upper laser level 4S3/2, however, requires a green pump laser, which leads to a rather low quantum defect for conversion to 1.73 μm. An alternative to Er3+ lasers includes lasers based on 3H4-3F4 laser transition in Thulium (Tm) around 1.5 μm, which are normally self-terminating due to an unfavorable lifetime ratio. However, such a laser action has been successfully demonstrated in the CW regime by quenching the long lived lower laser level through energy transfer to a Holmium 5I7 state and cascade lasing of the 5I7-5I8 transition at 2.06 μm in Tm, Ho:YLF (Thulium, Holmium: Yttrium Lithium Fluoride). See R. C. Stoneman and L. Esterowitz, “Continuous—wave 1.50-μm thulium cascade laser”, Optics Letters, Vol. 16, No 4, (1991).
It is well known that Holmium trivalent ions (Ho3+) are capable of producing stimulated emission at several wavelengths across the infrared spectrum. Laser emission from Ho3+ is normally associated with the 5I7-5I8 transition. Apart from this customary transition at 2.06 μm, other higher level transitions in Holmium can be very difficult to lase using standard excitation techniques such as flashlamps, which, being broad-band, require upper laser levels with long fluorescent lifetimes and a small non-radiative decay rate. Yet, some of the most interesting laser transitions, such as the transition at 1.67 μm, originate on levels characterized by lifetimes that are short compared with those of the lower levels. This effectively turns the transition into a three level laser system, resulting in self-terminating laser action. The 1.67 μm transition in Holmium occurs between the 5I5 and 5I7 levels or manifolds. The long lifetime of the lower level 5I7 (17 ms in Barium Yttrium Fluoride (BYF) and 14 ms in YLF) compared to that of the upper laser level 5I5 (50 μs in BYF and 20 μs in YLF), combined with the rapid nonradiative decay rates between the 5I5 and closely spaced 5I6 manifolds, in general produces conditions unfavorable for lasing. See K. M. Dinndorf, “Energy transfer between thulium and holmium in laser hosts,” Ph.D. Dissertation, MIT, (1993).
One solution for achieving laser action as a result of the 5I5→5I7 transition under such conditions without cooling is through use of linear down-conversion with pulsed resonant pumping, whereby the upper laser level is directly excited by a narrow band source with a frequency tuned to match an absorption line that is dynamically connected to the upper level of the desired transition. With direct excitation to the upper laser level, a population inversion can be created even from levels where long fluorescence lifetimes are not available, thus circumventing the limitation suffered by broadband excitation techniques. Furthermore, in certain conditions, advantage can be taken of cascaded processes whereby laser oscillation between intermediate levels is exploited for increasing a rate of transition to the upper level of a lower lying manifold, thus achieving inversion on the next, otherwise unfavorable laser transition. This removes the requirement for the long fluorescent lifetime and a small nonradiative decay rate that is otherwise imposed upon the upper laser level since population can be transferred to the desired level through the cascade laser process, with resultant multiwavelength sequential emission. As described below, stimulated emission at 1.67 μm was previously achieved in Ho:YLF, but under such conditions that severely limit prospects of further energy and power scaling to levels needed for the applications mentioned above.
The possibility of obtaining laser emission from a solid state laser doped with Ho3+ at 1.67 μm depends to a great extent on which host material is chosen. An important requirement for efficient conversion is that the fluorescence lifetime of the intermediate states and, primarily of the upper laser level should be long. Since the lifetimes of these states are often governed largely by nonradiative decay to lower lying states, it is necessary that the optical phonon energies of the host material be relatively small and/or orbital coupling of an ion to a lattice be relatively weak. These are also characteristics, which will permit fluorescence and stimulated emission to occur at longer wavelengths in the infrared spectrum. Therefore, fluorides, BaY2F8 (BYF) and LiYF4 (YLF) may be selected as laser host materials, where these conditions are satisfied. Other materials of interest are Ho3+ doped fluorozirconate ZBLAN glass, Ho:NaYF4 (Ho:NYF) and Ho:KY3F10 (Ho:KYF). An ability to sustain laser oscillation between two given levels is enhanced in fluorides over oxides in many cases because multiphonon decay rates in fluorides are generally lower. A reduction in upper state multiphonon decay rate generally means a larger product of upper state lifetime with stimulated emission cross-section, and therefore a lower threshold.
The long lifetime of the 5I7 energy manifold for most fluoride materials limits the repetition rate of transitions terminating on that level. There are several methods known from the prior art for efficient depletion of the long-lived lower laser level 5I7, which could lead to a substantial reduction of the effective lifetime of that level. One such approach is described in the U.S. Pat. No. 5,070,507 to Douglas W. Anthon, in which the lifetime of the lower laser level 5I7 of Ho3+ in BaY2F8 (BYF) is selectively quenched by addition of a small amount of such co-dopants as Praseodymium (Pr3+) and Europium (Eu3+). Anthon describes laser gain materials based on garnets such as Yttrium-Aluminum-Garnet (YAG), Gadolinium-Gallium-Garnet (GGG), and Yttrium-Aluminum Oxide (YALO) doped with high concentrations of Holmium ions (>15% atomic) and a much lower Pr3+ concentration (˜0.01%). An increase in Holmium concentration allowed efficient lasing of the 5I6-5I7 transition at 2.94 μm while the Pr3+ ion selectively quenched the lifetime of the terminal laser level, thereby breaking the bottleneck of the normally self-terminating laser transition.
Another method for efficient depletion of the 5I7 level can be accomplished by proper selection of the pump wavelength where the laser material exhibits not only ground state absorption but also excited state absorption from the 5I7 level. Pumping from a terminal laser level 5I7 of the 1.67 μm transition will create conditions similar to a four-level laser and potentially allow such a laser to be scaled to higher average powers, higher repetition rates or possibly even to the CW regime. CW operation of Ho doped fluoride fiber laser based on normally self-terminating transition at 2.9 μm with the 5I7 manifold as a terminal laser level was reported by L. Wetenkamp, “Efficient CW operation of a 2.9 μnm Ho3+-doped fluorozirconate fiber laser pumped at 640 μnm,” Electron. Letters, Vol. 26, (1990). The CW regime of lasing has been established with both pump wavelengths: 640 nm and 750 nm. A Holmium ion exhibits a number of matching resonant ground state and excited state absorption transitions between equally spaced energy levels. For example, a two-step pumping at 640 nm excites Holmium ions from 5I8 to 5F5 and from 5I7 to 5F3 energy levels, while 750 nm pumping allows excitation of Holmium ions from 5I8 to 5I4 and from 5I7 to 5S2 energy levels. Excited state absorption at both pumping wavelengths removes the population from the 5I7 manifold thereby effectively reducing the lifetime of the lower laser level. Two-step absorption is also described by A. M. Tabirian, “New, efficient, room temperature mid-infrared laser at 3.9 μm in Ho:BaY2F8 and visible Pr:LiYF4 laser for holography,” Ph.D. Dissertation, Physics Department/School of Optics, UCF, (2000), in which resonant pumping of Ho:BYF and Ho:YLF at high power densities at 750 nm results in strong depletion of the 5I7 population.
In Ho:LiYF4 (Ho:YLF), laser action at 1.67 μm is observed using short pulse resonant pumping of the 5S2 manifold by a frequency-doubled Nd:glass laser. See L. Esterowitz, R. C. Eckard, and R. E. Allen, Appl. Phys. Lett., 35, 236, (1979) and U.S. Pat. No. 4,321,559 to L. Esterowitz. By lasing the 5S2-5I5 transition at 1.39 μm, the excited state population could be directly transferred to the intermediate level 5I5, which serves as the upper level for a subsequent laser transition. In this manner, both the 5S2-5I5, 5I5-5I7 (1.392 μm, 1.673 μm) and 5S2-5I5, 5I5-5I6 (1.392 μm, 3.914 μm) cascade transitions were successfully lased at room temperature. Operation at these wavelengths has been limited, however, by a need to tune the laser pump to the absorption peak of the 5S2 manifold, near 535 nm. This wavelength matches up poorly with most readily available lasers, which was one of the factors precluding practical application of such cascade lasers. Accordingly, numerous approaches to identifying a better pump source for Ho3+ fluorides have been described in the art as shown by the following references: A. M. Tabirian, “New, efficient, room temperature mid-infrared laser at 3.9 μm in Ho:BaY2F8 and visible Pr:LiYF4 laser for holography,” Ph.D. Dissertation, Physics Department/School of Optics, UCF, (2000), and A. M. Tabirian, S. C. Buchter, H. P. Jenssen, A. Cassanho, H. J. Hoffman, “Efficient, room temperature cascade laser action at 1.4 μm and 3.9 μm in Ho:BaY2F8”, CLEO'99, Technical Digest 391, (1999) and U.S. Pat. No. 6,269,108 to A. M. Tabirian.
In one proposed pumping scheme, a commonly available Q-switched, frequency doubled Neodymium (Nd):YAG laser at 532 nm was used for off-peak pumping of Ho:BYF crystal with high levels of dopant concentration, which were chosen for maximizing the resonant pump absorption. Cascade laser action at 1.4 μm and 3.9 μm was demonstrated in 10% Ho:BYF with low thresholds and near-theoretical quantum efficiency. Another pumping scheme allowed the 3.9 μm energy to be scaled over 30 μmJ while achieving 14.5% slope efficiency by employing a direct resonant pumping of the upper laser level with a free running pulsed Cr:LiSAF laser tuned to 890 nm.
Such a system was based on high concentration crystals selected only for the purposes of optimization of pump absorption in end pump geometry in a relatively weak band around 890 nm. It was not realized at that time, however, that the high concentrations are crucial for the efficient laser action at 3.9 μm due to favorable combination of two nonradiative energy transfer processes: efficient cross relaxation populating the upper laser level and upconversion depleting the lower laser level of the 3.9 μm transition. Crystals with high Holmium concentrations, however, are not capable of producing efficient laser oscillation at the single wavelength of 1.67 μm. The main reason for this incapacity is a strong cross-relaxation in the host material that leads to a very fast build-up of population on the lower laser level and therefore, to self-termination of laser action. Moreover, in order to utilize higher concentration crystals for efficient generation of 1.67 μm laser radiation, special techniques of pumping, cascade lasing and co-doping would be required. Lasing of Holmium doped fluorides in general and BYF in particular at 1.67 μm has not been described by the prior art. Additionally, the high concentration samples described in the prior art would not allow efficient lasing at 1.67 μm as needed for practical applications.